With imaging and video applications driving an ever increasing appetite for wireless bandwidth, the need for spectrally efficient technologies that utilize available whitespace is apparent. Fast hopping, multiple-in and multiple-out (“MIMO”), and cognitive radio networks further exacerbate the problem, requiring full concurrent cooperation and tight closed loop control between transceivers across a flexible range of frequencies. Rapid development of new wireless signaling methods and standards also demands flexible hardware that can adapt to these changes.
Radio Frequency (“RF”) duplexers and diplexers are great assets in these systems, as they allow bidirectional communication either on the same band or over adjacent bands over the same path, enabling a transmitter and a receiver to share an antenna while transceiver communicating with minimal round-trip latency. Duplexers and diplexers preferably isolate the transmitter and receiver such that the receiver and transmitter do not load each other, the noise from the transmitter does not corrupt the receive signal, and the receiver is not desensitized (or damaged) by the high-power transmitted signal.
Previous diplexers rely on high-Q frequency selective filters to provide isolation, and/or ferrite structures such as circulators, preventing their integration in CMOS and increasing cost, size, and weight. Alternative approaches, using “electrical balance,” employ high-Q transformers to try to overcome this limitation, but this severely limits tunability to a narrow range of frequencies. And, the use of resonant, transformer structures to generate cancellation of the transmit signal at the receiver input suffers from intrinsic losses, degrading transmitter efficiency, and receiver noise performance. Finally, all of these approaches are narrow band and largely un-tunable, only providing isolation between receiver and transmitter across a frequency range of less than one octave.
An advantageous RF front would be fully integrated on chip with a single antenna port, would support simultaneous reception and transmission of RF signals, and would provide significant flexibility (i.e., multiple octaves) in center frequency and bandwidth of both receiver and transmitter. An active duplexer circuit capable of significant signal and noise isolation was demonstrated, however, the transmit power was limited to only 10's of microwatts. A truly useful system should meet these same requirements of isolation, integration and flexibility, while transmitting four orders of magnitude more power. No such system exists today.